1. Field of the Invention
The present application relates generally to a photo-optically controlled differential variable voltage attenuator and, more specifically, to a differential amplifier system incorporating a selectable electroluminescent differential photocell attenuator array which is used to vary an input gain of a differential amplifier system as a function of the output voltage.
2. Description of the Related Art
It is well known in audio frequency compression/limiting amplifiers to vary the gain of the amplifier relative to the amplitude of the audio signal being amplified. The gain is usually reduced when the input signal is excessively large and increases when the input signal becomes smaller. The gain of most compression and limiting amplifiers is controlled by a control voltage which is utilized to vary the amplification of one or more amplifier stages within the total amplifier system. The level sensing control voltage is usually developed from a rectified portion of either the input voltage (for limiting applications) or the output voltage of the amplifier (for compression applications). The control voltage is used to vary the bias of another amplifier stage so that the amplification or gain of the second amplification stage decreases when the input signal increases.
In general, compressors and limiters are amplifiers whose gain can be automatically varied as a function of the signal level; the higher the signal, the lower the gain. In the recording of electronic sound impulses, for example, music recording, some type of automatic limiting control system is required to prevent over-modulation of the recording medium or of the audio channel in use. A level control is effected by utilizing varying bias techniques within audio amplifiers, but this increases the distortion in the gain controlling amplifier stage because the optimum linear operating state of the amplifier is constantly being shifted.
An important aspect of signal limiting circuits is that the change in gain should not be audible to the listener as distortion of the signal or as an undesirable modulation of the signal envelope. This means that the circuit should have fast response times to audio signal peaks for efficient control of the amplifier system. When the bias signal is varied to either increase or decrease the audio amplification level, proper consideration of the attack time and release time constants of the components involved is required to obtain a rapid increase or decrease in amplification.
The present invention provides a gain adjusting amplifier which attenuates or compresses signals utilizing a differential compression circuit. The present invention also provides a compression or attenuation amplifier having compression controlled by detecting varying light levels of light sources driven by the input signal, output signal or some other signal.
The present attenuation differential gain adjusting amplifier system combines an electro-optical differential attenuator network, a differential audio amplifier, a differential control amplifier, and variable frequency/amplitude light sources. One or more photoresponsive cells is placed in proximity to one or more luminescent sources. The light sources are connected to respond to the input signals, or to the output signals, or some other signal. These light sources respond to the amplitude and/or the frequency of the signal to produce a light signal. The light signal is detected by the photoresponsive cells and cause the photoresponsive cells to provide a signal that is fed to the differential control amplifier. The control amplifier provides a control signal to the audio amplifier, which thereby varies the gain of the amplifier. The audio amplifier also receives the input audio signal and generates an output which is varied according to the changing gain of the circuit to effect a compression or limiting of the input signal.